Optical fiber sensors have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, glucose, inorganic ions, and hydrogen ions, in liquids and in gases. Such sensors are based on the recognized phenomenon that the absorbance or luminescence of certain indicator molecules is specifically perturbed in the presence of certain analytes. The perturbation in the absorbance and/or luminescence profile can be detected by monitoring radiation that is reflected or emitted by the indicator molecule when it is in the presence of a specific analyte. The targeted analyte is generally a part of a solution containing a variety of analytes.
Duofiber optical sensors have been developed that position an analyte-sensitive indicator molecule in the light path of a sensor tip. The indicator molecule is typically housed in a sealed chamber whose walls are permeable to the analyte. The sealed chamber is submerged in the analyte-containing solution. The sensor tip includes a pair of optical fibers. The term "duofiber" refers to the number of fibers in the sensor tip. In a duofiber sensor, one fiber transmits electromagnetic radiation, termed measuring signal, from a signal-generating component to the indicator molecule. The other fiber transmits the reflected or emitted light, termed indicator signal, from the sensor tip to a signal-measuring component that measures the indicator signal intensity. The configuration of the optical fibers between the signal-generating component, the sensor tip, and the signal-measuring component describes the optical fiber distribution system for the sensor system.
Although there are two common types of sensor systems: absorption and luminescent, the present invention is used in conjunction with a luminescent system. There are two types of luminescent sensor systems: phosphorescent and fluorescent. Such systems operate on the concept of measuring the luminescent intensity or lifetime of the radiation emitted by the excitation of the analyte-sensitive molecule. The signal generating component irradiates the indicator molecule with light at a wavelength band corresponding to a region of analyte-dependent absorbance by the indicator molecule. Luminescent emission from the indicator molecule is then measured by the signal-measuring component. The ambient analyte concentration is determined by known techniques as a function of the measured luminescent emission.
The optical fiber distribution system is an integral part of the optical fiber luminescent sensor system. The distribution system is made up of optical fibers, connectors, and components. The distribution system directs the measuring signal from the signal-generating component to the sensor tip and also directs the emitted indicator signal from the sensor tip to the signal-measuring component. In a sensor using a luminescent monitoring technique, the analyte concentration is determined by analyzing the emitted indicator signal over a specified time period.
The efficiency and reliability of a sensor system largely depends on its optical fiber distribution system. Although current optical fiber technology may not provide a one hundred percent signal transfer at fiber connection points, the signal reduction at optical fiber connections should be ascertainable and controllable. Variability in analyte concentration measurements that may be related to the optical fiber distribution system arise from fiber loss, fiber coupling inefficiency, fiber concentration, and response to noise, either random or periodic, produced by a variety of internal and external sources.
In current medical applications, it is desirable that the fiber distribution system be relatively small, flexible, and highly efficient. The size requirement becomes more crucial as in situ blood gas monitoring techniques are developed. For example, a blood gas catheter or sensor may be inserted into and left in a patient's body for a long period of time to provide continuous monitoring of specific conditions. The catheter tip includes the analyte-sensitive indicator molecule. For the patient's comfort, the catheter tip should be as small as possible. To accommodate this desirable size characteristic, a single fiber extending to the catheter tip is desirable. The remainder of the distribution system is then sized in proportion to the catheter tip fiber for maximum efficiency.
In a single-fiber sensor system, a single optical fiber carries the measuring signal to the indicator molecule, as well as carries the emitted indicator signal from the indicator molecule. One useful characteristic of a single-fiber system is that it is reducible to nearly one-half the size of the duofiber system at the sensor tip. However, a single-fiber sensor presents problems related to the small amount of light a single fiber, as well as the related distribution system, can carry, and the ability of the system to distinguish indicator signals from measuring signals that are reflected at imperfect fiber connections. The former problem is especially prevalent in analog-based sensors. Medical sensors are generally of this type. In analog-based sensors, the intensity of the signal produced at the sensor rather than the mere existence of the signal, as in a digital system, is significant. Each change in signal intensity that is not traceable to a constant in the distribution system will be attributed to a parameter in the monitoring process. Thus, the optical fiber distribution system must be highly predictable and reliable in order to provide useful monitoring results.
One known distribution system for a single-fiber sensor system includes lengths of optical fiber, a dividing connector, a mixing connector, a transmitting connector and a tip connector. The optical fiber lengths are of first and second diameters, the second diameter being larger than the first diameter and being substantially equal to the diameter of the sensor tip fiber. The dividing connector connects at least three intermediate fibers of the first diameter to the signal-generating component to thereby receive intermediate signals. The mixing connector connects a mixing fiber of the second diameter to the intermediate fibers to thereby receive the intermediate signals and blend them into a single mixed signal. The transmitting connector connects a transmitting fiber of the first diameter to the mixing fiber to thereby receive a portion of the mixed signal. The tip connector connects the transmitting fiber to the sensor tip fiber to thereby transmit the mixed signal to the sensor tip, and connects an indicator fiber of the first diameter to the sensor tip to thereby transmit a portion of the resulting indicator signal returned from the sensor tip to the signal-measuring component.
Previous PO.sub.2 modules used microfilters, non-cooled L.E.D.'s and a 2.times.2 optical coupler combination. Because of the small amount of phosphorescent return signal, the previous system provided a level of directivity (or cross-talk) that resulted in minimal performance. The major disadvantages were: low power output from the L.E.D.; poor signal-to-noise values; and a large amount of L.E.D. light corrupting the small amount of sensor phosphorescent return signal.